A method for dissolving and treating natural polymers and its application

Abstract

A broad-spectrum dissolution treatment method for natural polymers belongs to the field of polymer processing technology. This method uses a deep eutectic solvent as the solvent, dissolving at 60-80°C for 4-6 hours. The deep eutectic solvent is an aromatic hydrogen bond donor-acceptor deep eutectic solvent composed of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). The hydrogen bond donor is one of aromatic carboxylic acids, aromatic phenols, aromatic alcohols, and aromatic amides, and the hydrogen bond acceptor is at least one of benzyltrimethylammonium chloride and benzyltriethylammonium chloride. This invention utilizes a DES composed of aromatic HBA and HBD, which exhibits broad-spectrum solubility and excellent solubility for various natural polymers, overcoming the limitation of traditional DES's single applicability. This DES effectively shortens the dissolution time for poorly soluble polymers while improving their solubility. It is highly adjustable, with performance controlled by changing the HBD / HBA combination or ratio.

Description

Technical Field

[0001] This invention relates to the field of polymer processing technology, specifically to an efficient dissolution treatment method and its application for natural polymers based on deep eutectic solvents. Background Technology

[0002] Traditional natural polymer materials hold immense potential in fields such as biofuels and advanced carbon materials. However, due to their strong intermolecular hydrogen bonds, high crystalline ratio, and strong thermal stability, they are often difficult to dissolve efficiently in conventional organic solvents or aqueous systems, thus limiting their subsequent processing (such as spinning, casting, electrospinning, and 3D printing). Existing solvents such as NaOH / urea aqueous solutions have limited solubility and are easily degraded; water / ethanol mixed solvents are inefficient; ionic liquids are costly, difficult to recycle, and toxic; and organic solvents such as LiCl / DMAc require stringent pretreatment and generate hazardous waste. While some existing systems for dissolving natural polymers (such as ionic liquids and strong acid-base systems) have shown some effectiveness, they generally suffer from strong corrosivity, high toxicity, high cost, high viscosity, and difficulties in solvent regeneration, limiting their industrial application. Deep eutectic solvents, on the other hand, have gradually become an important research direction for green processing of polymer materials in recent years due to their lower energy consumption in preparation, higher degree of freedom in structural design, and good biocompatibility.

[0003] Deep eutectic solvent systems are eutectic mixtures formed by mixing hydrogen bond donors (HBD) and hydrogen bond acceptors (HBA) in a certain molar ratio. Their core characteristics are: the melting point of the mixture is much lower than that of the two components individually (typically below 100°C, even liquid at room temperature), and it exhibits low volatility and high solubility similar to ionic liquids (ILs), but is simpler to prepare, lower in cost, has better biocompatibility, and is renewable and biodegradable. Therefore, it is hailed as a "next-generation alternative to green solvents."

[0004] Traditional deep eutectic solvents, such as those for choline chloride (HBA) and urea (HBD), primarily rely on the ion-dipole interaction between the quaternary ammonium cation of choline and the carbonyl oxygen of urea, as well as the hydrogen bonding between the NH4+ of urea and the hydroxyl oxygen of choline. Through competition and substitution, they disrupt the original hydrogen bond network within the polymer and stabilize the separated molecular chains in solution through solvation. However, this traditional single-mechanism approach is unsuitable for the complex and diverse structures of polymers, resulting in poor solubility. Therefore, developing deep eutectic solvents with high solubility for polymers with different structures is extremely important for the green dissolution and functional preparation of natural polymer materials. Summary of the Invention

[0005] The purpose of this invention is to provide an aromatic hydrogen bond donor-acceptor deep eutectic solvent that provides efficient and broad-spectrum dissolution of natural polymers.

[0006] Another objective of this invention is the application of natural polymer materials after dissolution treatment.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A method for efficient and broad-spectrum dissolution of natural polymers, characterized by using a deep eutectic solvent as the solvent and dissolving at 60-80°C for 4-6 hours, wherein the deep eutectic solvent is an aromatic hydrogen bond donor-acceptor deep eutectic solvent composed of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), wherein the hydrogen bond donor is one of aromatic carboxylic acids, aromatic phenols, aromatic alcohols, and aromatic amides, and the hydrogen bond acceptor is at least one of benzyltrimethylammonium chloride (BTMAC) and benzyltriethylammonium chloride (BTEAC).

[0009] Furthermore, the aromatic carboxylic acid is benzoic acid, salicylic acid, p-hydroxybenzoic acid or p-aminobenzoic acid, the aromatic phenol is phenol, p-cresol, m-cresol or p-hydroxyphenyl ether, the aromatic alcohol is benzyl alcohol or phenylethanol, and the aromatic amide is benzamide, p-hydroxybenzamide or o-hydroxybenzamide.

[0010] More preferably, HBA is composed of benzyltrimethylammonium chloride (BTMAC) and benzyltriethylammonium chloride (BTEAC) in a molar ratio of 1:1.

[0011] Furthermore, the molar ratio of HBA to HBD is 1:0.8~3, forming a liquid deep eutectic solvent at 40~60℃.

[0012] An aromatic hydrogen bond donor-acceptor deep eutectic solvent, composed of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), characterized in that: the hydrogen bond donor is one of aromatic carboxylic acids, aromatic phenols, aromatic alcohols, and aromatic amides, and the hydrogen bond acceptor is at least one of benzyltrimethylammonium chloride (BTMAC) and benzyltriethylammonium chloride (BTEAC).

[0013] Furthermore, the aromatic carboxylic acid is benzoic acid, salicylic acid, p-hydroxybenzoic acid or p-aminobenzoic acid, the aromatic phenol is phenol, p-cresol, m-cresol or p-hydroxyphenyl ether, the aromatic alcohol is benzyl alcohol or phenylethanol, and the aromatic amide is benzamide, p-hydroxybenzamide or o-hydroxybenzamide.

[0014] More preferably, HBA is composed of benzyltrimethylammonium chloride (BTMAC) and benzyltriethylammonium chloride (BTEAC) in a molar ratio of 1:1.

[0015] Furthermore, the molar ratio of HBA to HBD is 1:0.8~3, forming a liquid deep eutectic solvent at 40~60℃.

[0016] Existing traditional DES rely on ion-dipole interactions and hydrogen bonding to disrupt the original hydrogen bond network within the polymer through competition and substitution, and then stabilize the separated molecular chains in solution through solvation. However, the traditional single-action mechanism is not suitable for the complex and diverse structures of polymers, resulting in limitations in its applicability and varying solubility for different polymers.

[0017] The HBA and HBD used in this invention both contain aromatic rings, and a strong π-π stacking occurs between the benzyl group of HBA and the aromatic ring of HBD. Furthermore, a cation-π interaction occurs between the quaternary ammonium salt cation of HBA and the aromatic ring of HBD, collectively forming a continuous, more electron-delocalized joint aromatic interface. This interface more effectively enters the hydrophobic region of the natural polymer through hydrophobic effects and π-π stacking, increasing the contact area and initial affinity between the solvent and the substrate. This provides favorable conditions for subsequent hydrogen bond attack. The joint aromatic interface disrupts the hydrophobic stacking and van der Waals forces between polymer chains, and enhances hydrogen bonding and electrostatic interactions through the formed strong electron-delocalized structure. This allows the deep eutectic solvent to interact with the natural polymer compound at multiple points, resulting in a multi-dimensional synergistic effect and significantly improving the solubility of natural polymers with different structures.

[0018] Furthermore, the -CH2- group in the benzyl group (-CH2-C6H5) of HBA selected in this invention has an electronic regulation effect, isolating the electronic inductive effect of the benzene ring on the quaternary ammonium nitrogen atom, thus maintaining the positive charge and hydrogen bond acceptance of the nitrogen atom at a very strong level. Simultaneously, -CH2- acts as a connector, allowing the benzene ring and HBD to better form π-π stacking, resulting in a more continuous and electronically delocalized co-aromatic interface. The method for preparing the above-mentioned aromatic hydrogen bond donor-acceptor deep eutectic solvent is characterized by comprising the following steps:

[0019] (1) Weigh each component according to the predetermined molar ratio;

[0020] (2) Add each component to the container and stir evenly with ultrasonic assistance at room temperature;

[0021] (3) Heat and keep the temperature constant in the range of 20℃~100℃ while stirring until a uniform and transparent liquid deep eutectic solvent system is formed.

[0022] Furthermore, the frequency of the ultrasound is 40~50 kHz, and the ultrasound duration is 10~15 min.

[0023] Furthermore, the heating temperature is 40~80℃, the stirring speed is 250~350r / min, and the heat preservation and stirring time is 15~30min.

[0024] The application of the above-mentioned deep eutectic solvents in dissolving natural polymer materials.

[0025] Furthermore, the natural polymeric materials include polysaccharides, proteins, composite materials, and functionalized biomacromolecules.

[0026] Furthermore, the polysaccharide natural polymers include, but are not limited to, chitosan, carboxymethyl cellulose, alginate, agar, and pectin.

[0027] Furthermore, the protein-based natural polymers include, but are not limited to, gelatin, silk fibroin, and soy protein.

[0028] Furthermore, the composite materials include, but are not limited to, cellulose / chitosan composite membranes and nanocellulose composites.

[0029] Furthermore, the functionalized biomacromolecules include quaternary ammonium chitosan, hydroxypropyl chitosan, etc.

[0030] The present invention has the following technical effects:

[0031] This invention utilizes a DES composed of aromatic HBA and HBD, exhibiting broad-spectrum solubility and excellent solubility for various natural polymers such as cellulose, chitosan, and starch. This overcomes the limitations of traditional DES with their single applicability. The DES effectively shortens the dissolution time for poorly soluble polymers while simultaneously improving their solubility. It also offers high adjustability, allowing performance to be controlled by altering the HBD / HBA combination or ratio. The polymers dissolved and regenerated using the DES of this invention can be used to prepare thermal insulation materials, conductive aerogels, and controlled-release drug films. Attached Figure Description

[0032] Figure 1 DES images prepared in Examples 1, 2 and 3 of this invention.

[0033] Figure 2 Image of the mixture of DES and cellulose prepared in Example 1.

[0034] Figure 3 Images of cellulose before and after precipitation and regeneration.

[0035] Figure 4 XRD patterns of microcrystalline cellulose before and after regeneration with DES solvent. Detailed Implementation

[0036] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above description.

[0037] Example 1

[0038] An aromatic hydrogen bond donor-acceptor deep eutectic solvent, composed of hydrogen bond donor HBD and hydrogen bond acceptor HBA, wherein the hydrogen bond donor is p-hydroxybenzoic acid and the hydrogen bond acceptor is benzyltrimethylammonium chloride, and the specific preparation steps are as follows:

[0039] (1) Weigh out benzyltrimethylammonium chloride and p-hydroxybenzoic acid in a molar ratio of 1:2;

[0040] (2) Each component was added to a high-temperature ultrasonic reactor and ultrasonically treated for 15 minutes at a frequency of 40 kHz. The reaction temperature was controlled at 50℃ using a water bath circulation system. The viscosity of the resulting deep eutectic solvent was measured to be about 21% lower than that of the sample directly mixed without ultrasonic treatment, which is beneficial for the subsequent dissolution process.

[0041] Comparative Example 1:

[0042] In the preparation of the deep eutectic solvent, phenyltrimethylammonium chloride (PTMAC), which also has aromaticity, was used instead of benzyltrimethylammonium chloride (BTMAC) in Example 1.

[0043] Although PTMAC also possesses a benzene ring structure, the benzene ring is directly attached to a positively charged quaternary ammonium nitrogen atom, resulting in a high electron density environment around the benzene ring. This steric hindrance makes it difficult for the aromatic ring of HBD to form aromatic conjugation with PTMAC. Furthermore, the benzene ring interacts with the strongly electron-withdrawing -N... + The direct connection of (CH3)3 leads to a decrease in the π electron cloud density of the benzene ring, weakening the ability of π-π stacking between it and HBD, resulting in an unsatisfactory overall effect of the deep eutectic solvent.

[0044] Comparative Example 2

[0045] In the preparation of the deep eutectic solvent, tetraphenylphosphine chloride (TPPC), which also has aromaticity, was used instead of benzyltrimethylammonium chloride (BTMAC) in Example 1, while the rest remained unchanged.

[0046] The results showed that no homogeneous liquid structure was formed in the mixed system, but rather a suspension of solid particles of HBD encapsulating HBA, indicating that no deep eutectic solvent was formed after the two were mixed at the above temperature.

[0047] TPPC contains more benzene rings, but the benzene rings surrounding the central phosphorus atom form a large steric hindrance. This spatial shielding hinders the approach of HBD to the interaction site. Furthermore, its own crystal structure is extremely stable with low degrees of freedom. When mixed with HBD, the intermolecular forces cannot destroy its highly ordered crystal structure, and a eutectic mixture cannot be formed.

[0048] Dissolution experiment of highly crystalline cellulose by deep eutectic solvent:

[0049] An α-cellulose sample with a crystallinity index (CrI) of 75% and a degree of polymerization (DP) of 1200 was selected. 5 g of the sample was weighed and added to 100 mL of the deep eutectic solvent prepared in Example 1. A staged heating dissolution strategy was adopted.

[0050] Pre-swelling stage: The temperature is controlled at 40℃, and the mixture is left to stand and gently stirred for 30 minutes to allow the cellulose molecular chains to fully absorb the solvent and expand and open the chains.

[0051] Dissolution stage: Heat to 65℃ and continue stirring for 2 hours to promote the breaking of some cellulose chain segments and solvent penetration. Stir until saturated and the cellulose is fully dissolved.

[0052] The final solution system was a transparent and homogeneous liquid. The transmittance was measured to be 92% using a 660 nm UV-Vis spectrophotometer, indicating that the highly crystalline cellulose was effectively dissolved in this solvent.

[0053] Cellulose solubility determination

[0054] 1. Weigh 18.5693g of benzyltrimethylammonium chloride and 27.6241g of p-hydroxybenzoic acid (molar ratio 1:2) using an electronic analytical balance, and prepare a DES solution according to the steps in Example 1.

[0055] 2. Weigh 1.0g of α-cellulose using an electronic balance, and slowly add it to a high-temperature ultrasonic reactor containing DES under heat preservation and stirring conditions. Keep the reactor warm and stir continuously for 24 hours to saturate the DES.

[0056] 3. Take out the prepared saturated DES solution and let it stand in a 50℃ constant temperature oven for 6 hours until the solution separates into layers.

[0057] 4. Add a certain mass of clear, saturated DES solution to a clean beaker placed on an analytical balance with its mass set to zero, and record the mass as m0.

[0058] 5. Add 20 mL of 50°C anhydrous ethanol (antisolvent) to the above beaker. At this point, a large amount of flocculent material can be observed to be produced.

[0059] 6. Transfer the above solvent to a 50 mL centrifuge tube, centrifuge at 3400 rpm for 4 min, and repeat three times. Transfer the resulting solid to a petri dish with a mass of m1.

[0060] 7. After drying in a vacuum drying oven at 50 degrees Celsius for 6 hours, weigh the plate containing the solid and record the mass as m2.

[0061] The solubility is calculated using the following formula:

[0062]

[0063] Following the same dissolution steps described above, the same α-cellulose samples were dissolved using the DES prepared in Comparative Example 1 and Example 2. In addition, the same dissolution experiment was conducted with DES containing different HBD compositions while keeping the HBA constant. The results are shown in Table 1.

[0064] Table 1:

[0065]

[0066] The cellulose dissolved in DES is re-precipitated, corresponding to the original cellulose sample; this precipitated cellulose is called regenerated cellulose. Figure 3 From left to right, the images show the precipitation of DES solutions of cellulose in hot ethanol in Comparative Example 1 (left), Comparative Example 2 (middle), and Example 1 (right).

[0067] Figure 4 The image shows the XRD characterization of cellulose before and after dissolution. It can be seen that the crystal form of the molecules changes before and after dissolution, which can verify the dissolution of cellulose.

[0068] Example 2

[0069] An aromatic hydrogen bond donor-acceptor deep eutectic solvent, composed of hydrogen bond donor HBD and hydrogen bond acceptor HBA, wherein the hydrogen bond donor is benzamide and the hydrogen bond acceptor is benzyltriethylammonium chloride, and the specific preparation steps are as follows:

[0070] (1) Weigh out benzyltrimethylammonium chloride and p-hydroxybenzoic acid in a molar ratio of 1:1;

[0071] (2) Each component was added to a high-temperature ultrasonic reactor and ultrasonically treated for 15 minutes at a frequency of 40 kHz. The reaction temperature was controlled at 50℃ using a water bath circulation system. The viscosity of the resulting deep eutectic solvent was measured to be about 23% lower than that of the untreated sample, which is beneficial for the subsequent dissolution process.

[0072] Dissolution of high-degree-of-deacetylation chitosan by deep eutectic solvents

[0073] Chitosan powder with a degree of deacetylation (DD) of 75%, a crystallinity index (CrI) of 70%, and a molecular weight (MW) of 100 kDa was selected and added to the deep eutectic solvent prepared in Example 2 to prepare a chitosan solution. The dissolution temperature was controlled at 60°C, and the solution was stirred until dissolved. Under the same conditions, chitosan was dissolved using conventional DES, DES with a single aromatic structure, and DES prepared in Comparative Example 1 and Example 1. In addition, the same dissolution experiment was conducted with DES of different HBD compositions while keeping the HBA constant. The results are shown in Table 2.

[0074] Table 2:

[0075]

[0076] The DES in Example 1 reduced the dissolution time of chitosan by about 40% compared with Comparative Example 1. The chitosan was completely dissolved, the solution was free of particulate impurities, had good fluidity, and was suitable for subsequent processing.

[0077] Example 3

[0078] Compared to Example 1, the hydrogen bond acceptor was composed of benzyltrimethylammonium chloride (BTMAC) and benzyltriethylammonium chloride (BTEAC) in a molar ratio of 1:1, with the remaining components and steps being the same as in Example 1.

[0079] Example 4

[0080] The DES prepared in Example 3 was used to dissolve different polymers, including cellulose, chitosan, starch, and gelatin. The dissolution results are shown in Table 3.

[0081] The cellulose sample used was α-cellulose with a crystallinity index (CrI) of 75% and a degree of polymerization (DP) of 1200.

[0082] The degree of deacetylation (DD) of chitosan is 75%, the crystallinity index (CrI) is 70%, and the molecular weight (MW) is 100 kDa.

[0083] The starch is corn starch, with a crystallinity index (CrI) of 35%, a degree of polymerization (DP) of 800, and a molecular weight (MW) of 140 kDa.

[0084] The gelatin has a molecular weight (MW) of 60 kDa and contains 50% hydrophilic amino acids.

[0085] Table 3:

[0086]

[0087] BTMAC and BTEAC have different alkyl chain lengths. After compounding, the hydrophilic-lipophilic balance of each DES can be finely adjusted, so that it can generate optimal affinity with various micro-regions on the surface of natural polymer materials, thereby achieving more thorough solubilization, shortening the dissolution time, and having higher solubility for natural polymers.

[0088] Material application and processability:

[0089] After dissolving natural polymer materials using the DES system provided by this invention, the following material preparation and processing can be performed:

[0090] (1) Prepare conductive aerogel using cellulose and chitosan dissolved by the above DES:

[0091] The cellulose / chitosan mixed solution prepared in Example 4 (cellulose to chitosan volume ratio of 1:1) was adjusted to a solid content of 8%, and a polyaniline nanowire dispersion with a diameter of approximately 50 nm was added. After uniform mixing, in-situ oxidative polymerization was carried out. The polymerization conditions were stirring at room temperature for 12 h to form a uniform and dense composite film. The film was then transferred to anhydrous ethanol and subjected to a gradient displacement at volume ratios of 1:1, 3:1, and 5:1 (6 h each time), until the film was completely immersed in anhydrous ethanol. The film was then displaced with a 50% ethanol / 50% tert-butanol (v / v) solution for 12 h, followed by sequential displacement with 80% and 100% tert-butanol for 6 h each. The film was then pre-frozen at -80°C for 24 h, then maintained at -50°C for 24 h, then heated to -20°C and maintained for 12 h, and finally heated to 25°C and maintained for 12 h to obtain an aerogel film. The conductivity of the obtained aerogel film was measured using the four-probe method to be 5.1 × 10⁻⁶. -3 With a conductivity of S / cm, it exhibits excellent electrical conductivity and is suitable for flexible electronic devices and biosensors.

[0092] (2) Preparation of controlled-release membrane:

[0093] 5% (by mass) of natural tea polyphenols was added to the DES-cellulose solution in Example 4, and after thorough mixing, a film was prepared using a casting method. After drying, the film was placed in phosphate-buffered saline (PBS) for release performance testing. The test results showed:

[0094] In an acidic environment with pH=2.0, the tea polyphenol release rate of the membrane was 18% after 24 hours, which was slow and showed good controlled-release characteristics.

[0095] In a neutral environment at pH 7.4, the release rate significantly increased to 63%, indicating that the membrane has pH-responsive release capability. This controlled-release membrane is suitable for drug delivery and smart packaging applications.

[0096] (3) Preparation of thermal insulation materials:

[0097] The DES-cellulose solution from Example 4 was allowed to stand for 10 min, and then deionized water was added to induce cellulose precipitation and gel formation. The gel was distilled at 80°C and 8 kPa for 2.5 h. The distilled gel was then soaked in deionized water for 24 h, with the deionized water being replaced every 6 h. Finally, it was freeze-dried to obtain the thermal insulation material. The material's thermal insulation performance was tested: the porosity was 91.4%, and its thermal conductivity was 0.20 W / (m·K). This is because DES forms a hydrogen-bonded complex with cellulose upon dissolution. During the antisolvent-induced gelation process, the aromatic structure of DES creates steric hindrance, and its compatibility with the antisolvent induces the formation of micron-nano-scale composite pores, improving the pore uniformity of the material, adjusting the heat conduction path, and reducing thermal conductivity, thus achieving excellent thermal insulation performance.

Claims

1. A broad-spectrum dissolution treatment method for natural polymers, characterized in that, A liquid deep eutectic solvent is used as the solvent to dissolve cellulose, chitosan, starch, or gelatin at 60-80°C for 4-6 hours. The liquid deep eutectic solvent is an aromatic hydrogen bond donor-acceptor liquid deep eutectic solvent composed of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA). The hydrogen bond donor is one of aromatic carboxylic acids, aromatic phenols, aromatic alcohols, and aromatic amides. The hydrogen bond acceptor is at least one of benzyltrimethylammonium chloride (BTMAC) and benzyltriethylammonium chloride (BTEAC). The aromatic carboxylic acid is benzoic acid, salicylic acid, p-hydroxybenzoic acid, or p-aminobenzoic acid. The aromatic phenol is phenol, p-cresol, m-cresol, or p-hydroxyphenyl ether. The aromatic alcohol is benzyl alcohol or phenylethanol. The aromatic amide is benzamide, p-hydroxybenzamide, or o-hydroxybenzamide. The molar ratio of HBA to HBD is 1:0.8-3. A liquid deep eutectic solvent is formed at 40-60°C. The liquid deep eutectic solvent is used to dissolve cellulose, chitosan, starch, or gelatin.

2. The broad-spectrum dissolution treatment method for natural polymers as described in claim 1, characterized in that: The HBA is composed of benzyltrimethylammonium chloride (BTMAC) and benzyltriethylammonium chloride (BTEAC) in a molar ratio of 1:

1.

3. A broad-spectrum dissolution treatment method for natural polymeric substances as described in claim 1 or 2, characterized in that: The preparation method of the liquid deep eutectic solvent includes the following steps: (1) Weigh each component according to the predetermined molar ratio; (2) Add each component to the container and stir evenly with ultrasonic assistance at room temperature; (3) Heat and keep the temperature constant in the range of 20℃~100℃ while stirring until a uniform and transparent liquid deep eutectic solvent system is formed.

4. The broad-spectrum dissolution treatment method for natural polymers as described in claim 3, characterized in that: The ultrasonic frequency is 40~50 kHz, and the ultrasonic time is 10~15 min.

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